1. Field of the Invention
The present invention relates to an image sensing apparatus and its control method.
2. Description of the Related Art
An exemplary mechanism for electronic zoom processing in CCD cameras is shown in FIG. 1. In FIG. 1, reference numeral 100 denotes a lens, reference numeral 101 denotes a charge-coupled device (CCD), reference numeral 102 denotes a correlated double sampling circuit (CDS), and reference numeral 103 denotes a clamping circuit (CLP). Further, reference numeral 104 denotes an analog-digital converter (A/D), reference numeral 105 denotes frame memory, reference numeral 106 denotes a zoom controller, reference numeral 108 denotes an image compensation unit, and reference numeral 107 denotes an image output.
In this mechanism, an optical image passes through the lens 100 and is formed on a photo-sensing surface of the CCD 101, which is an image sensing device. The optical image formed on the photo-sensing surface of the CCD 101 is converted into photo-charges in a two-dimensionally arranged photoelectric converter and sequentially transferred to the output. The correlated double sampling circuit 102 eliminates a CCD-specific reset noise from the output signals of the CCD 101 to generate sampled-and-held video signals that are reset noise free.
The clamping circuit 103 performs clamping at the dark level, and the AD converter 104 converts the input analog signals into digital signals. The frame memory 105 is memory for recording data on all pixels of one frame. The zoom controller 106 reads out only a partial area around the center of the CCD when, for example, 2× zoomed image data is desired.
Recently, CMOS image sensors have been increasingly in use because they are inexpensive, require no complicated timing generation circuits, and operate with a single power supply while consuming less power. Furthermore, the CMOS image sensors have a characteristic which CCD image sensors do not have, i.e., the ability of capturing only arbitrary areas of the CMOS image sensors as an image.
Description will be given of a high image quality electronic zooming of a CMOS image sensor capable of reading out arbitrary areas (see Japanese Patent Laid-Open No. 2001-78081). FIG. 2 is a conceptual view of electronic zoom operations for CMOS image sensors. Reference numeral 201 in FIG. 2 denotes readout method in normal mode, and reference numeral 202 in FIG. 2 denotes readout method in zoom mode. In normal mode, for example, a value obtained by adding together four pixel values of a solid image sensing device is read out as a pixel value for one pixel. In the range inside a bold line of 201, pixel values for shaded pixels are read out, wherein each pixel value is obtained by adding together four pixel values of that pixel and pixels at the right, lower right, and directly below that pixel. That is, pixel values for 4×4 pixels are read out from the range of 8×8 pixels.
On the other hand, in zoom mode, pixel values of an area of continuous 4×4 pixels (a shaded part) around the center of the range of 8×8 pixels inside the bold line are directly read out without addition. Then, the center portion in the bold line can be displayed in an enlarged form. In addition, since the number of read-out pixels is the same as that in normal mode and no pixel padding by signal processing is required, high image quality can be provided in electronic zooming.
When photo-charges are accumulated in a CMOS sensor capable of block readout, the timing of starting the accumulation is controlled on a line basis. Therefore, the time of accumulating the photo-charges does not align between the lines. This temporal misalignment between the lines corresponds to the time required to read out one line. This time required to read out one line may be calculated by the following equation.
The readout time per line=HBLK×α+Skip×β+the number of horizontal pixels×reference clock time  Equation (1)
(α and β are values determined by the manner of addition in the vertical direction.)
As an example, FIGS. 3A to 3C show driving that involves adding together and averaging two pixel lines in the vertical direction, where α is 2 and β is 1. That is, the readout period per line is determined by summing the following: time HBLK required to transfer the first line, time Skip required to skip the second line, time HBLK required to transfer the third line, and the time required to transfer pixel values in the horizontal direction obtained by adding together and averaging the first and third lines.
The time required to transfer the pixel values in the horizontal direction also depends on the reference clock time (the drive frequency). That is, the readout period per line varies with the manner of addition in the vertical direction and the drive frequency. As a result, the misalignment of the time to start accumulation between the top and bottom of a screen varies with changes in these drive mode conditions.
FIGS. 4A and 4B are diagrams for describing the misalignment of the accumulation period according to the readout period per line. Comparing FIGS. 4A and 4B, the readout period per line in FIG. 4B is longer. Herein, the driving as in FIG. 4A with the shorter readout period per line will be referred to as a “drive mode A”, and the driving as in FIG. 4B with the longer readout period per line will be referred to as a “drive mode B.” In drive mode A, the misalignment of the accumulation period between the top and bottom of the same display screen is smaller than that in drive mode B.
Referring to FIG. 5, description will be given of the case where the drive mode is switched in taking moving images, using EVF (Electronic View Finder) and so forth. In FIG. 5, reference numeral 501 denotes the sum of the accumulation period and the readout period for pixel values in a frame driven in drive mode A. In FIG. 5, for lines for which readout for one frame has finished, accumulation of photo-charges for the following frame is started. Therefore, the readout period for the preceding frame overlaps the start of accumulation for the following frame. Reference numeral 502 denotes the sum of the accumulation period and the readout period when the drive mode is switched from drive mode A to drive mode B at time t1. The switching of the drive mode is performed during the VBLK (vertical blanking) period after the pixel values in the frame 2 are read out. That is, the accumulation start timing corresponding to drive mode A is set for lines before the drive mode switching, and the accumulation start timing corresponding to drive mode B is set for lines after the drive mode switching.
In the example shown in FIG. 5, the driving is performed in drive mode A before time t1, at which point the driving is switched to drive mode B. Since the reset starting time for the frame 3 is before time t1, the gradient due to the misalignment of the accumulation start timing in the period before time t1 corresponds to drive mode A. However, the drive mode is switched to drive mode B at time t1, so that the readout period becomes longer in contrast to drive mode A. As a result, the misalignment of the reset start timing for lines for which the reset is started after time t1 corresponds to drive mode B, causing a different gradient of misalignment. Then, trying to maintain the frame rate would cause a difference in the accumulation period between the top and bottom of the same display screen for the frame 3.
Thus, as in the above example, when the readout period for a frame overlaps the accumulation period for the following frame, switching the drive mode causes a difference in the accumulation period in the following frame, thereby reducing the quality of the output image.